This application relates to a ground fault detection method and device for a system where a high common mode choke condition exists.
Modern aircraft electrical systems receive electrical power from three-phase generators which are mechanically connected to the turbines of the aircraft engines. In typical systems the electricity produced by a generator may contain variations due to electrical noise or other factors. Such electricity may not be suitable for use with sensitive on-board electronics found in most aircraft. In order to condition the electricity, most applications connect the generator output to an inverter/conditioner which conditions the power to be in an acceptable form. A side effect of the conditioning is that a high common mode choke may be needed. Among other known effects, the common mode choke prevents current from exceeding a certain value, even in the case of a ground fault.
A ground fault may occur for any number of reasons such as the mechanical touching of wires, failure of components, or improper connections. A phase to ground fault occurs where a direct electrical connection is created between one phase of a multiphase system and electrical ground. This results in a phase imbalance and may disrupt electrical systems and may cause physical damage to the electrical system.
Various methods have been employed in an attempt to detect a ground fault so that the faulty generator may be isolated from the system and potential damage from the imbalance prevented. One scheme to detect a ground fault compares the current on each phase of the electrical system to a threshold, and when the current exceeds the threshold a phase to ground fault is determined to be present. Such a method will operate in any system without a common mode choke since the direct link to ground will short circuit the load and all the power will flow to ground, resulting in a large current spike. These systems measure the current output from the generator, and when the current on a single phase increases by a certain amount (typically 5 to 6 amperes) a phase to ground fault is determined to exist. The scheme may be inoperable when a high common mode choke is present since the common mode choke prevents an increase in current.